The Mail app on iOS supports multiple email accounts and you get to easily choose the default email to send stuff from by default. To change default email account on iPhone, iPad and iPod Touch devices, do as follows:
1. Open the Settings application
2. Scroll down to the list of default applications and tap to open the Mail, Contacts, Calendars settings
3. Scroll down to the Mail settings and tap the Default Account option
4. Choose the email account on your iPhone, you want to set as default account and send and share messages, media files and other stuff using this email address by default.